CHARACTERIZATION OF PHOSPHATE FILMS ON ALUMINUM SURFACES

A thin layer of phosphate conversion coating was formed on pure alumin um in a commercial zinc-manganese phosphating bath. A number of surfac e analytical techniques were used to characterize the phosphate thin f ilms formed after immersion times ranging from 30 s to 10 min. The coa ting contained mainly a crystalline structure with dispersed micromete r-scale cavities. The major constituents of the phosphate film were zi nc, phosphorus, and oxygen; a small amount of manganese was also detec ted. Based on these results, a three-stage mechanism was proposed for the formation and the growth of phosphate conversion coatings on alumi num. Electrochemical impedance spectroscopy was used to evaluate the c orrosion performance of phosphated and uncoated aluminum samples in 0. 50 M Na2SO4 and 0.10 M H2SO4 solutions. Both types of samples exhibite d a passive state in the neutral solution and general corrosion behavi or in the acid solution.